Decarburizing molten steel

ABSTRACT

The application describes a method for decarburizing molten steel. It comprises the steps of introducing oxygen and recirculated diluent gas into a vessel containing a liquid bath of metal, in a manner which precipitates a reaction between carbon within the metal and oxygen. The diluent gas is carbon monoxide or a mixture of carbon monoxide and carbon dioxide. It is obtained by collecting and treating the gas which exits from the vessel.

United States Patent Fulton et al.

[ 1 Mar. 14, 1972 [54] DECARBURIZING MOLTEN STEEL 3,252,790 5 1966 Krivsky ..75/ [72] Inventors: James C. Fulton; Sundaresan Ramacham 3,336,132 8/l967 McCoy ..75/60 X both fights, FOREIGN PATENTS OR APPLICATIONS [731 Assign: Allegheny Ludlum Cwlmmfim, Pitt- 883,958 12/1961 Great Britain ....75/60 sburgh 944,479 12/1963 Great Britain ..75/60 [22} Filed: Aug. 29, 1969 Primary Examiner-L. Dewayne Rutledge [21] Appl 854,226 Assistant ExaminerG. K. White AttorneyRichard A. Speer, Vincent G. Gioia and Howard R. [52 vs. C] ..75/60, /59 Berkenstock, [51] Int. Cl ...C2lc 5/28 [58] Field of Search ..75/60, 59 ABSTRACT The application describes a method for decarburizing molten [5 6] References Clted steel. It comprises the steps of introducing oxygen and recircu- UNITED STATES PATENTS lated diluent gas into a vessel containing a liquid bath of metal, in a manner which precipitates a reaction between carbon 2,81 1,435 Bannister 8t 81. X the meta] and xygen The diluent gas is carbon monox- 2,969,232 1/1961 chtufchef 75/ 60 ide or a mixture of carbon monoxide and carbon dioxide. It is 3,003,365 10/1961 Bridges 6 6 "75/60 obtained by collecting and treating the gas which exits from 3,046,107 7/1962 Nelson et al. 75/60 x the vcSSeL 3,058,823 10/1962 Churcher ..75/60 3,215,523 1 1/1965 Richardson ..75/60 10 Claims, 1 Drawing Figure m ARGO/V ARGO/V flg OXYGEN SUPPLY .SEPARATOR SUPPLY a COMPRESSOR m S TORA GE CHAMBER M 00 T0 c0 ,3 REDUCER COOL/N6 AND 5 /2 CL EA N/NG CHAMBER COMPRESSOR s TOR/1 6E CHA M)? FA N Q U 5 6 7 a lo 5 5 5 5 I O 9 C 2 DECARBURIZING MOLTEN STEEL The present invention relates to a decarburizing process and more particularly to a decarburizing process which utilizes oxygen and recirculated diluent gas.

Decarburizing is an essential part of present-day steelmaking processes. It is generally performed by blowing oxygen into a vessel, e.g., an electric furnace, an open hearth furnace, or a basic oxygen furnace (BOF), containing a liquid bath of metal, in a manner which precipitates a reaction between car bon within the metal and oxygen.

A shortcoming of decarburizing processes is the oxidation of metallic values; e.g., chromium, manganese and iron, from the liquid bath. This oxidation is due to the use of a surplus amount of oxygen, i.e., oxygen in excess of that required for carbon removal, which reacts with metallic values forming metallic oxides. Formation of these oxides often necessitates a subsequent operation to recover the lost metallic values. Slags formed during the decarburization of corrosion resistant steels are often treated with a reducing agent; e.g., ferrosilicon, so as to effect a return of chromium to the metal.

It would appear that the oxidation of metallic values could be curtailed by reducing the amount of available oxygen. The most obvious method for accomplishing this would be to simply reduce the oxygen input flow rate. This, however, diminishes the mixing caused by the input of the gas which in turn reduces the oxygen consumption rate and the rate of oxygen reaction with carbon. An alternative approach reduces the amount of oxygen but maintains total gas flow rates by substituting a diluent gas for the abstracted oxygen. Thus, the oxygen input rate can be varied without reducing the rate of oxygen reaction with carbon, by introducing a diluent gas with the oxygen.

A second shortcoming of decarburizing processes is fume production which generally necessitates the use of expensive precipitators to clean exiting gaseous products. During decarburization, a large quantity of brown fume is given off when oxygen reacts with carbon to form carbon monoxide and carbon dioxide.

We have developed a decarburizing process which introduces oxygen and a diluent gas comprised of recirculated exiting gas, into the decarburizing vessel. It precludes most of the metallic oxidation which occurs when oxygen is used without a diluent gas, significantly reduces oxygen usage and substantially eliminates wasteful fume production and expensive cleaning procedures while utilizing a valuable gas produced during the decarburizing process.

It is accordingly an object of this invention to provide a novel decarburizing process.

It is a further object of this invention to provide adecarburizing process which recirculates and employs exiting gas as input diluent gas.

The foregoing and other objects of the invention will be best understood from the following description, reference being had to the accompanying drawing wherein:

The figure is a schematic diagram of apparatus which can be used to carry out the process of this invention.

The process of this invention is applicable to both plain carbon and alloy steels, e.g., stainless steels and silicon steels. It comprises the steps of introducing oxygen and recirculated diluent gas into a vessel containing a liquid bath of metal, in a manner which precipitates a reaction between carbon within the metal and oxygen. The diluent gas is preferably carbon monoxide but could be a mixture of carbon monoxide and carbon dioxide. It is obtained by collecting and treating, e.g., cooling and cleaning, the gas which exits from the vessel. Carbon monoxide is preferred over carbon dioxide since it has a lower thermal capacity than does carbon dioxide and, therefore, will not draw as much heat from the liquid bath and since it is a stronger diluent as it reacts with oxygen whereas carbon dioxide reacts with carbon. The ratio of diluent gas to oxygen should be at least 1:4 to insure attainment of a substantial reduction in metallic oxidation. It constantly changes' with variations in conditions such as liquid bath temperature and metal composition and is determined stoichiometrically by measuring the rates of carbon removal and oxygen input In decarburizing steels it is often necessary to remove certain elements; e.g., silicon and aluminum, before the carbon level can be reduced to low values. In such cases, the rate at which silicon, aluminum, etc., are being oxidized can be measured and included in determining the oxygen input so that sufficient oxygen is provided to accomplish decarburization at the desired rate as well as oxidation of the other elements, e.g., silicon and aluminum, also intended to be removed.

Decarburization of plain carbon steels is generally, but not necessarily, initiated by introducing only oxygen into the vessel. It is preferable to fon'n a slag rich in iron oxide since iron oxide buildup in the slag is desirable for phosphorus removal. Additionally, the heat that is drawn from the bath to raise the temperature of diluent gas could be used to melt scrap. When the iron oxide content of the slag is at least 10 percent, preferably 15-20 percent, it is beneficial to introduce diluent gas. Further increases in the iron oxide content of the slag enlarges the yield loss of iron without significantly changing the phosphorus removal. In addition, iron oxide behaves in a very corrosive manner which accelerates the rate at which furnace linings wear out.

Alloy steels are presently decarburized by blowing pure oxygen into the vessel. The oxygen exothermically reacts with elements; e.g., chromium, within the bath, thereby raising the bath temperature. This invention, as stated earlier, introduces gases; e.g., carbon monoxide and oxygen, in proportions which precludes most of the metallic oxidation as well as the accompanying increase in the temperature of the bath. Wellknown equilibrium relationships point out that it is essential at atmospheric pressure to reach very high melt temperatures at the end of the oxygen blow, in order to decarburize to low carbon levels and maintain relatively high chromium contents. These high temperatures are generally not attained with the proportioned decarburizing gases of this invention. It is, therefore, desirable to lower the partial pressure of carbon monoxide in the vessel. A lowering of the partial pressure of carbon monoxide changes the equilibrium relationships and shifts the attainable end point carbon to low levels at lower temperatures without requiring a reduction in the chromium content of the bath. Reduction in the partial pressure can be accomplished by reducing the pressure in the vessel and/or introducing argon into the vessel. Argon can be introduced with oxygen, with oxygen and carbon monoxide or with oxygen, carbon monoxide and carbon dioxide.

Illustrative recirculating apparatus for carrying out the process of this invention is schematically shown in the figure. It comprises a vessel 1 containing a molten steel bath 2, a closed hood 3, a lance 4 passing through closed hood 3 and into vessel 1, valves 5, gas cooling and cleaning chamber 6, suction fan 7, sensors 8, vent 9, compressor 10, vent ll, storage chamber 12, carbon dioxide reducer l3, storage chamber 14, argon supply 15, argon separator 19, compressor 16, oxygen supply 17, and bin 18. The lance 4, can assume any of the well-known lance configurations. Cooling and cleaning chamber 6 could comprise a heat exchanger and bag-type gas cleaner or any other suitable cooling and cleaning equipment. Suction fan 7 draws the exiting gas from the vessel into the recirculating apparatus. It also renders the apparatus, and of course the method of this invention, suitable for a vacuum degassing operation which removes nitrogen and hydrogen from the molten steel bath as well as lead and other volatile tramp elements while the metal is in vessel 1. Sensors 8 determine gas composition, pressure, and temperature. Compressor l0 pumps the gas into storage chamber 12. Storage chamber 12 has a gas composition, pressure and temperature sensor built in. Vent 11 makes a high calorific value gas available for other heating requirements in the steelmaking plant. Carbon dioxide reducer 13 contains coke or graphite to reduce carbon dioxide to carbon monoxide. Compressor l6 pumps diluent gas into lance 4. Bin 18 is for feeding slag forming ingredients, e.g., lime. Argon separator 19 isolates argon from the other gases so that argon can be stored and supplied to the system as required.

The following example emphasizes several aspects of the invention.

TABLE I ygen is introduced into a vessel containing a liquid bath of metal, to react with carbon contained within the metal, the improvement which comprises; collecting carbon monoxide and carbon dioxide gases which have exited from the vessel,

passing at least said carbon monoxide gas through a cleanin chamber, compressing at least said car on monoxide gas an recirculating at least said carbon monoxide gas by introducing same with said oxygen into said vessel.

2. A method according to claim 1 including the step of reducing carbon dioxide gas to carbon monoxide gas by Gas analyses v0 Flow rate of percent) Bath Melt analyses, percent total gases Blow time ten? s.c.l.m./ton Input Input Heat (minutes) C Mn S1 Cr Fe charged 00 02 A study of Table 1 reveals that heat A which was decarburized to the extent of 0.43 percent C with oxygen underwent a chromium loss of about percent whereas heat B which was decarburized to the extent of 0.45 percent C with oxygen and carbon monoxide did not undergo any observable chromium loss. Table I also shows that carbon monoxide constituted a high percentage of the gas in the blowing stream, throughout the decarburizing operation for heat B. This is because the carbon content of the liquid bath was initially only 0.65 percent and because the chromium content was relatively high. If the carbon content of the liquid bath was initially higher, e.g., 3 percent, the percentage of carbon monoxide in the blowing stream could have been lower, e.g., 20 percent. Higher oxygen levels and lower carbon monoxide levels are tolerable when the liquid bath has a higher carbon content. The maximum percentage of oxygen allowable in the blowing stream without appreciable metal oxide formation decreases as carbon content in the liquid bath is lowered.

It will be apparent to those skilled in the art that the novel principles of the invention disclosed herein in connection with specific examples thereof will suggest various other modifications and applications of the same. lt is accordingly desired that in construing the breadth of the appended claims they shall not be limited to the specific examples of the invention described herein.

We claim:

1. In the method of decarburizing molten steel wherein oxpassing carbon dioxide gas through a carbon dioxide to carbon monoxide reducer.

3. A method according to claim 1 wherein said recirculated gas containing carbon monoxide and said oxygen are introduced into said vessel at a respective ratio of at least about I14.

4. A method according to claim 1 adapted to produce plain carbon steel, wherein said vessel contains a liquid bath of metal and siag and wherein said recirculated gas containing carbon monoxide is not introduced until said slag contains at least about 10 percent iron oxide.

5. A method according to claim 4 wherein said recirculated gas containing carbon monoxide is not introduced until said slag contains between about 15 and 20 percent iron oxide.

6. A method according to claim 1 adapted to produce alloy steel and including the step of introducing argon into said vessel from a gas supply consisting essentially of argon.

7. A method according to claim 6 including the step of separating argon from said gases which have exited from said vessel for reintroduction into said vessel.

8. A method according to claim I adapted to produce alloy steel and including the step of reducing the pressure in said vessel.

9. A method according to claim 1 including the step of vacuum degassing the molten steel bath in said vessel after the bath has been decarburized.

10. A method according to claim 1 including the step of removing carbon dioxide from said gases which have exited from said vessel. 

2. A method according to claim 1 including the step of reducing carbon dioxide gas to carbon monoxide gas by passing carbon dioxide gas through a carbon dioxide to carbon monoxide reducer.
 3. A method according to claim 1 wherein said recirculated gas containing carbon monoxide and said oxygen are introduced into said vessel at a respective ratio of at least about 1:4.
 4. A method according to claim 1 adapted to produce plain carbon steel, wherein said vessel contains a liquid bath of metal and slag and wherein said recirculated gas containing carbon monoxide is not introduced until said slag contains at least about 10 percent iron oxide.
 5. A method according to claim 4 wherein said recirculated gas containing carbon monoxide is not introduced until said slag contains between about 15 and 20 percent iron oxide.
 6. A method according to claim 1 adapted to produce alloy steel and including the step of introducing argon into said vessel from a gas supply consisting essentially of argon.
 7. A method according to claim 6 including the step of separating argon from said gases which have exited from said vessel for reintroduction into said vessel.
 8. A method according to claim 1 adapted to produce alloy steel and including the step of reducing the pressure in said vessel.
 9. A method according to claim 1 including the step of vacuum degassing the molten steel bath in said vessel after the bath has been decarburized.
 10. A method according to claim 1 including the step of removing carbon dioxide from said gases which have exited from said vessel. 